An optical communication system using an optical fiber line such as an optical fiber uses an optical pulse line monitoring apparatus in order to detect breakage of the optical fiber line or determine a fracture position. The optical pulse line monitoring apparatus utilizes generation of backscattered light with the same wavelength as the wavelength of light propagating through an optical fiber line and propagation of the backscattered light in the opposite direction.
That is, when an optical pulse is input to an optical fiber line as test light, the optical pulse continues to generate backscattered light until the optical pulse reaches a fracture point. A reflected light with the same wavelength as that of the test light is output from a facet of the optical fiber line to which the optical pulse has been input. The fracture position on the optical fiber line can be determined by measuring the duration of the backscattered light. An optical time domain reflectometer (OTDR) is a typical monitoring apparatus based on the above-described principle.
However, for the passive optical network (PON) optical split line, the optical pulse line monitoring apparatus has difficulty individually identifying branched optical fibers positioned on a user apparatus side with respect to an optical splitter or the states of an optical device (reflective filter), a splitter, a fiber connection component, and the like which are connected to the optical fiber line.
For example, a trunk optical fiber extended from a telecommunications carrier facility building is split into a plurality of optical fibers by an optical splitter. Test light is also uniformly distributed among the optical fibers (hereinafter referred to as “branched optical fibers”) provided by the optical splitter. In this case, upon returning to an input end of the trunk optical fiber, reflected lights from branched optical fiber lines overlap at the optical splitter. This prevents which of the branched optical fibers is broken to be determined based on observed OTDR waveforms.
Thus, the existing optical pulse line monitoring apparatus is basically effective on a single optical fiber line and fails to be applied directly to optical split lines. Thus, a technique has been proposed which enables the optical pulse line monitoring apparatus to be applied to the optical split lines (see Non Patent Document 1 and Patent Document 1).
According to Non-Patent Document 1, an optical filter that reflects test light is installed upstream of user apparatuses as a termination filter so that an OTDR apparatus with a high resolution can measure the intensity of reflected light from each user apparatus. The technique has been reported to achieve an accuracy of 2 m, in terms of distance resolution, in branched optical fibers located downstream of the optical splitter.
However, this technique is limited to a level at which a defective branched optical fiber line is identified and a defective position is isolated, for example, which of the user apparatus and the optical fiber line is defective is determined, and fails to determine where in the branched optical fiber the defect is occurring.
On the other hand, Patent Document 1 proposes that an optical splitter comprise a wavelength multiplexer/demultiplexer of a diffraction grating type based on an array waveguide utilizing multiple-beam interference of light so that a wavelength variable light source can switch the wavelength of test light to select a test target optical fiber line. According to the proposed method, the wavelength of the wavelength variable light source is swept, an optical reflection processing section detects the wavelength of reflected light, and the wavelength of the test light is set based on the wavelength of the reflected light. Thus, the optical fiber lines can be individually monitored in association with the wavelength of the test light.
However, optical splitting devices with a wavelength routing function, typified by a wavelength multiplexer/demultiplexer of a diffraction grating type based on an array waveguide, are generally expensive. It is difficult, in terms of costs, to use such an optical splitting device for an access network optical system accommodating a large number of subscribers. Moreover, such optical components depend significantly on temperature and need addition of a temperature adjustment function. This leads to the need for high costs when the system is constructed.